Novel iron-chromium-beryllium alloy system

ABSTRACT

THE PRESENT INVENTION RELATES TO SELECTED IRON-BERYLLIUMCHROMIUM ALLOY SYSTEMS CONTAINING MINOR ADDITIONS OF AT LEAST ONE STRENGTH-INDUCING ADDITIVE SELECTED FROM THE GROUP CONSISTING OF TUNGSTEN, NIOBIUM, CARBON, AND YTTRIUM. THESE ALLOYS ARE CHARACTERIZED BY HIGH STRENGTH FROM ROOM TEMPERATURE TO TEMPERATURES UP TO AT LEAST 550*C. AND BY A STABLE, BODY-CENTERED, CUBIC STRUCTURE OVER THE FULL RANGE OF TEMPERATURE FROM ROOM TEMPERATURE UP TO THE MELTING POINT.

United States Patent 3,625,675 NOVEL IRON-CHROMIUM-BERYLLIUM ALLOY SYSTEM Fred C. Robertshaw and Roger J. Perkins, Cincinnati,

Ohio, assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Apr. 1, 1969, Ser. No. 812,360 Int. Cl. C22c 39/14 US. Cl. 75-126 2 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to selected iron-berylliumchromium alloy systems containing minor additions of at least one strength-inducing additive selected from the group consisting of tungsten, niobium, carbon, and yttrium. These alloys are characterized by high strength from room temperature to temperatures up to at least 550 C. and by a stable, body-centered, cubic structure over the full range of temperature from room temperature up to the melting point.

The present invention was made in the course of, or under, a contract with the US. Atomic Energy Commission.

SUMMARY OF THE INVENTION The present invention is based on the discovery that iron-base alloys containing prescribed concentrations of beryllium and chromium have significant strength at temperatures ranging from room temperature to at least 550 C. Although, as will be discussed, certain of the alloys have some resistance to oxidation, the alloys are primarily intended for service in non-oxidizing environments, including liquid metals such as sodium or inert gases.

The invention makes use of beryllium additions-up to its solubility limit to a selected iron-base alloy under equilibrium conditions at room temperature-4o aid in stabilizing the body-centered cubic structure of iron over the full range of temperature from room temperature to the melting point. Concentrations of from 2 to 8 atom percent beryllium are suitable for this purpose.

Of equal importance is the finding that beryllium has a significant strengtheningefiect up to temperatures of about 550 C. when present mainly in solid solution. The presence of beryllium in slight excess of solubility can result in the formation of a beryllium-bearing dispersed phase which, if present in small amounts, can actually contribute to the strength of the resultant alloys. Excessive beryllium containing dispersed phase leads to embrittlement.

The use of chromium in the range 515 atom percent (a/o) imparts strength and oxidation resistance to a reference Fe-8 a/o Be alloy. Chromium additions beyond this level result in excessive precipitation of an a phase during long-term exposure at temperatures in the range 400 to 550 0, resulting in impaired ductility near room temperature.

The addition of yttrium at the 1 a/o level has been found to effect a further increase in 550 C. strength.

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Yttrium up to this amount tends to promote a fine-grained structure and to provide an extra increment of strength due, it is generally thought, to the presence of a dispersed Y-Fe phase. The addition of refractory elements such as tungsten and niobium in amounts up to their approximate low temperature solubility limits in iron, along with carbon additions to effect the formation of carbide with the carbide-forming elements, results in a further increase in strength.

The alloys of this invention can be prepared either by powder metallurgy techniques or by melting and casting into ingot form under vacuum or under an inert atmosphere to avoid contamination and the formation of inclusions such as oxides or nitrides. Another feasible method of preparation is by air melting under suitable slag. The alloys described in this invention were prepared in a vacuum induction furnace under a pressure of -.S atmosphere of argon. Resultant alloy ingots were homogenized at -1l00 C. and then extruded at a temperature in the range 9501100 C., using an area reduction during extrusion of about 12-16 to 1. Short sections of the extrusion were then press forged at about 1000 C. in a direction perpendicular to the major axis of the extrusion to thickness (about 5-1 thickness reduction). The fiatted extrusion segments were then hot rolled to -0.030" (nominal) sheet at heating temperatures ranging from l000 down to 760 C. Rolling was conducted in a direction to the original extrusion direction. Following finish rolling, the sheet was given a stress relief anneal at 750 C. for 20 hours. Tensile and stress rupture specimens were machined from the resulting sheet and tested with results indicated in Tables I and II. Specimen dimensions included a 0.030-thick by 0.250"-wide reduced section within a 1.0-long gauge section.

Tensile and stress-rupture data for representative alloys within the scope of the invention are shown in Tables I and II. For purposes of comparison, various commercial alloys useful in the same range of temperature are also included. Except as indicated alloy compositions in this specification are in atom percent.

TABLE I [Tensile properties for iron alloys containing Be and other elements vs.

ferritic and austenitic stainless steels] Test tempera- Yield Ultimate Elon- I-Ieat Composition, ture, strength, strength, gation, No. Atom percent C. k.s.i. k.s.i. percent RT 70. 6 129. 2 19. 8 g 3 8 Master heat from which others were i 550 68 8 1 0 made. 3 Fe-lO Cr-8 Be-l Y 4 Fe-10 CB8 Be-l.5 RT 124.4 157.0 11.3 W-l Nb-0.l5 C. 550 72. 0 88. 2 14. 5 5 Fe-10 Cr-8 Bra-1.5 RT 136.0 2.5 1V; Nb- 0.15 C- 550 65. 7 85.8 13. 2 6 Fe-ll Cr-8 Al-.6 Y ggg 7 A181 430 SS 'I 39.0 74. 0 -31. 0 (Fe-17 Cr-. 44 C). 550 25. 0 37. 0 -40. 0 8 A181 304 SS RT 28. 3 85. 5 74. 6 (Fe-20 Cr- 9 Nl- 550 10. 1 53. 3 35. 0 1.8 Mn-.28 C).

TABLE II [Stress-rupture properties for Fe-Cr-Be alloys vs. ierritic and austenitic stainless steels] Test Prc- Posttemtest test pera- Test Elonhardhard- Heat ture, Atmos- Stress, time, gation, ness, ness, No. Composition, atom percent C phere p.s.i. hours percent DPH DPH Remarks 1 Fe-8 Be 550 irgon "v5. 1 Ruptured.

rgon 5. 1 324 Do. 2 Fe 10 or 8 Be 550 i A Air 2g, g s 24. 7 330 333 Do. rgon .6 21.8 353 353 Do. 3 Fe 10 Cr 8 Be 1 Y 550 i Air 45, 000 149. 5 24. 353 348 Do.

Fe 10 (Jr-8 Be-1.5 W-l Nb-.15 550 Air 45, 000 1, 034. 0 2. 7 366 354 No rupture. Fe-lO Cr-8 Be-1.5 W-l Nb-.15 550 Air 45, 000 1, 320. 5 1. 7 363 363 D0.

Air 20, 000 23. 9 Ruptured. 6 Fe-ll Cr-S A1-.6 Y 550 Air 15, 000 168.0 Do. 482 Air 1?, 38g. 0 Do. Air 4 0 .8 Do. 7.. AISI 430 SS (Fe-17 Cr-. 44 C) 566 Air 20: 000 113 D o s A151 304 ss (Fe-20 (Jr-9 Ni-1.8 Mn-. 28 0 550 21f 3:

* Elongation is post-test measurement from fillet to fillet (1.5) opposed to the standard 1 gage. Pt. gage strip loosened in test. b Data obtained from ASTM special publications, No. 228, Report on Elevated-Temperature Properties of Chromium Steel.

A comparison of the strengths of the several alloys 1-5 within the scope of this invention shows (1) that the addition of chromium to the binary Fe-Be alloy in amounts up to a/o has a strength-inducing effect, (2) that the addition of yttrium to the Fe-Cr-Be alloy in amounts up to 1 a/o also improves 550 C. strength, (3) that the addition of small amounts of tungsten-niobiumcarbon to the Fe-Cr-Be alloy further improves strength, and (4) that the addition of Y as well as tungsten-niobium-carbon tends to reduce the short-time properties but increase the long-time properties of these alloys. The difference in argon and air stress-rupture strengths in argon and air tests of alloys 2 and 3 is believed associated with internal oxidation in the air tests.

Another and somewhat more graphic way of showing the strength level of the alloys within the scope of this invention is to compare them with alloys 68, which are normally regarded as having utility in the same range of temperature. Comparing alloy 3 with alloy 6, it is seen that the essential difference resides in the substitution of Be for Al on an atom-for-atom basis. This substitution results in an alloy having ultimate strength higher by a factor of 23 over the range of temperature from room temperature to 550 C., and a similar or greater increase in stress-rupture strength at 550 C. The same pattern is evident for alloy 7, a commercial ferritic stainless steel, and alloy 8, a commercial austenitic stainless steel, both of which have been used as a material of construction for apparatus designed to operate at these temperatures. The significant benefit or improved strength in representative alloys 1-5 is self evident.

The results of heating sheet specimens of each of the experimental alloys in air at 550 C. for 97 hours is indicated in the weight gain data of Table III.

TAB LE III {Weight gain data for iron alloys containing Bo and other elements after heating in air at 650 C. 10197 hours] 1 Master heat from which alloys 3, 4, and 5 were made.

It is apparent that the addition of 10 a/o Cr to the Fe-B a/o Be alloy has markedly influenced gross oxidation resistance. The further addition of W, Nb, C, and Y has had little further effect in aiding oxidation resistance, at least in the this time interval.

For the sake of clarity, the Fe-Cr-Be-Y basic system and modifications which fall within the scope of claim 25 are expressed in whight percent as follows:

a/o w/o 1 Fe-S Be IO Cr 1 Y 86. 92 FG1.38 Be9.99 Cll.71 Y 2 Fe-S Be-5 C1'-1 Y-2 Mo 88$? Feasts 13:54.90 Cr-l. s Y3.62

O 3 Fe-8 Be-10 Cr-1 Y-2 Mo 83.48 I e-1.36 Beam (Dr-1.69 Y3.63 Mo 4 Fc8 Be-15 Cr-1 Y2 Mo 78.1418 Fe1.37 Be14.81 Cr1.69 Y3.65

O 5 Fe-8Be-1OCr-1 Y-2W 80. I e-1.32 Be9.52 (Ir-1.63 $5.74 5 Fe-8 Be-lO (Jr-.15 C 88.50 i e-1.395 Be-10.07 (Jr-.035 C 7 Fe-s Be-IO CH Y2 Nb 83.58 Fe-iae Beast (Jr-1.60 Y-a52 Nb 8 Fe-s Be-10 Cr1.5W-1 82.15 Fe-L335 Beast Cr5.12 W1.72

Nb.15 C Nb.035 G 9 Fe 8 Be 10 (Jr-1 Y1.5 80.62 Fla-1.33 Be9.58 (Jr-1.64 Y5.085

W-i Nb.15 c w-1.71 Nb.035 C What is claimed is:

1. An iron-base alloy consisting essentially of, in weight percent, 0.3-1.4 beryllium, 4.75 to 15.25 chromium, yttrium in an amount sufiicient to form a dispersed Y-Fe phase up to a maximum of 1.75 percent, and the balance iron.

2. A steel consisting essentially of, in weight percent, 0.3-1.4 beryllium, 4.7515.25 chromium, yttrium sufficient to form a dispersed Y-Fe phase, up to a maximum of 1.75 percent yttrium, a small amount of carbon up to a maximum of 0.10 percent, and a carbide-forming refractory metal selected from the group consisting of molybdenum, tungsten, and niobium up to a maximum of 3.65 percent molybdenum, 6.74 percent tungsten, and 3.52 percent niobium.

References Cited UNITED STATES PATENTS 1,713,766 5/1929 Marshall 75-126 G 1,943,347 1/1934 Smith 126 G 2,104,836 1/1938 Hessenbruch 75126 G 2,393,488 1/1946 Touceda 75-126 G 2,403,141 7/1946 Touceda 75--126 G 3,099,128 7/1963 Straumann 75 126 R 5 3,113,991 12/1963 Kleber 75126 G HYLAND BIZOT, Primary Examiner 

